JP 5-160348A discloses a conventional input protection circuit that protects internal circuits from destruction when a surge voltage is applied to an input terminal of a semiconductor integrated circuit. In this circuit, as shown in FIG. 13, an input terminal 1 is connected to a positive terminal of a comparator 3, which is an internal circuit, via a resistor 2, and to the collector of an NPN transistor, which is also and internal circuit, via a diode 4.
The emitter of the transistor 5 is connected to the ground, and the base is connected to other internal circuits (not shown). A negative terminal and an output terminal of the comparator 3 are also connected to other internal circuits (not shown), respectively. The diode 4 is disposed to prevent reverse flow of current to the input terminal 1 from the ground via the transistor 5 when a ground potential rises for some reason.
Between the input terminal 1 and the ground, a series circuit of a diode 6 in a forward-biased direction and a Zener diode 7 in a reverse-biased direction, and a series circuit of a diode 8 in a reverse-biased direction and a Zener diode 9 in a forward-biased direction are connected. That is, an input protection circuit 10 is constructed with diodes 6 to 9, and forms a semiconductor integrated circuit 11 together with other internal circuits.
When a surge voltage of positive polarity is applied to the input terminal 1, and a corresponding voltage applied to the Zener diode 7 exceeds a Zener voltage VZ, the Zener diode 7 breaks down and becomes conductive. Then, since a current flows from the input terminal 1 to the ground via the diode 6 and the Zener diode 7, a potential at the input terminal 1 is clamped to (VZ+Vf). Vf is a forward voltage of the diode 6. As a result, the comparator 3 and the transistor 5 can be protected from the surge voltage.
The above protection operation is desirable, as long as the diode 4 for the reverse flow prevention and the transistor 5 are both OFF. However, actually, for example, if an extremely high level of surge voltage of about 100V is applied to the input terminal 1, the diode 4 might turn on momentarily due to current leak or the like. In this case, the collector of the transistor 5 has substantially the same potential as at the input terminal 1. When the surge voltage exceeds an OFF withstand voltage Vceo, the transistor 5 would be broken down. The above problem also occurs likewise when, for example, a resistor for limiting an input current is disposed in place of the diode 4.
U.S. Pat. No. 6,385,021 discloses another conventional input protection circuit. In this circuit, as denoted with numeral 112 in FIG. 23, input terminals 101 and 102 of a semiconductor integrated circuit are connected to internal circuits 103 and 104, respectively. The internal circuits 103 and 104 are a transistor, an operational amplifier, or a comparator, for example. The input terminal 101 is connected to the anode of a diode 105 and the cathode of a diode 107. The input terminal 102 is connected to the anode of a diode 106 and the cathode of a diode 108.
The anodes of diodes 107 and 108 are connected to the circuit ground, and the cathodes of the diodes 105 and 106 are connected to the circuit ground via the drain and the source of an N-channel LD (lateral diffused) MOSFET 109. A Zener diode 110 is connected between the drain and the gate of FET 109, and a resistor 111 is connected between the gate and the source of FET 109.
A surge voltage of positive polarity may be applied to the input terminal 101 or 102. When a corresponding voltage applied to the Zener diode 110 exceeds a Zener voltage VZ, the Zener diode 110 becomes conductive, a terminal voltage develops in the resistor 111, and the FET 109 turns on. Then, since a surge current flows into the circuit ground via the FET 109 from the input terminal 101 or 102, the internal circuit 103 or 104 can be protected.
According to the input protection circuit 112, although protection operation is possible for application of a surge voltage of positive polarity, protection is impossible when a surge voltage of negative polarity such as field decay is applied in an IC such as an electronic control unit (ECU) mounted in a vehicle. The input protection circuit 112 may be modified as shown in FIG. 24 to protect the internal circuits from surge voltages of negative polarity. In FIG. 24, although showing a case of only one input terminal 101, an N-channel LDMOSFET 113 is connected between the circuit ground and the anode of the diode 107, and a Zener diode 114 and a resistor 115 are connected between the drain and the gate of an FET 113, and between the gate and the source of the same, respectively.
However, when the entire circuit is integrated into a single semiconductor integrated circuit chip (IC), if each circuit element is formed by PN junction isolation, a parasitic diode 107a is formed in the diode 107 for reverse flow prevention between terminals as shown in FIG. 25. Therefore, when a surge voltage (e.g., about −100 V) of negative polarity is applied to the input terminal 101, a surge current flows from the ground via the parasitic diode 107a. As a result, the FET 113 of the input protection circuit does not turn on.
In this case, since the parasitic diode 107a cannot withstand such a large current, it may be thermally broken down. Because of such circumstances, it is conventionally not easy to integrate electronic circuits including the protection circuit into an IC. As a result, external discrete elements are used as a protection countermeasure. Thus, the entire circuit becomes large and increases costs.